1. Field of the Invention
The present invention relates to a vehicle control system for controlling the running state of a vehicle. More particularly, the present invention relates to a vehicle control system for electronically controlling driving of a prime mover, steering and braking in a vehicle such as an automobile.
2. Description of the Related Art
As a vehicle control system, there was hitherto known an operation control system for controlling various control means related to a braking force, a driving force and a steering angle by an electronic control system, i.e., one master, in a concentrated manner (see, e.g., Patent Reference 1: JP-A-2003-263235).
Recently, aiming at an improvement of comfortableness in driving and safety of an automobile, a vehicle control system has intensively been developed in which driver's operations on an accelerator, a steering wheel and a brake are reflected on mechanisms for generating vehicle driving, steering and braking forces under electronic control without resorting to mechanical linkages. In such a vehicle control system, high reliability is required to ensure that the functions of driving, steering and braking the vehicle are in no way failed.
In other words, control realized with a mechanical mechanism has been replaced by control realized with an electrical mechanism. Typical examples are the so-called “Fly-by-Wire” in airplane control and “X-by-Wire” in automotive control. In these applications, a mechanical backup mechanism was initially provided to be ready for the event of a failure. However, the electrical mechanism has been required to have high reliability with a tendency to omit the mechanical backup mechanism.
Of the “X-by-Wire” (XBW system) for electrical control of an automobile, the “Steer-by-Wire” for electrical control of steering and the “Brake-by-Wire” for electrical control of braking are required to positively operate without malfunctions and to have high reliability. In particular, the “Steer-by-Wire” is required to have extremely high reliability because of the absence of a (failsafe) steering position that ensures safety in the event of a failure.
On the other hand, the merit of the “X-by-Wire” resides in that vehicle safety is improved by electronically controlling the steering and braking operations in an integrated manner. This point is represented by the widely-spreading concept “active safety”, i.e., the concept of enhancing safety of an automobile with the so-called vehicle stabilization control (see, e.g., Patent Reference 3: JP-A-10-291489).
In one known example of the vehicle control system aiming at high reliability, the reliability is improved by providing a dual-redundant master controller that has the functions of ABS (Antilock Brake System), TCS (Transmission Controlled System), etc. (see, e.g., Non-Patent Reference 1: Literature of First “X-by-Wire” Seminar, by D&M Nikkei Mechanical, etc., pp. 3-6, lower left picture in slide 12). Another example is intended to realize high reliability by designing a control module for a front wheel brake to continue the normal operation even when an error occurs (so-called “fail-operative”), and designing a control module for a rear wheel brake to stop the function when an error occurs (so-called “fail-silent”) (see, e.g., Patent Reference 2: JP-A-2002-347602).
As another type of vehicle control system, there is also known, for example, a system in which an operation amount applied from a driver to a maneuvering means, such as a brake pedal, is converted to an electric signal and the electric signal is transmitted to a control computer provided in a brake mechanism via a communication means, such as a CAN (Controlled Area Network), thereby performing electronic control.